Conventionally, in a load sensor measuring a surface pressure, wirings crossing each other at right angles are arranged on two surfaces of a sensing rubber that is a film shape, and a load detection is executed based on a change of a resistance of the sensing rubber at a crossing point where a load is applied by using a change of a thickness of the sensing rubber at the crossing point. However, since the load sensor is a simple matrix structure in which the wiring crossing each other at right angles in a matrix shape on the two surfaces of the sensing rubber and executes a load detection by using the simple matrix structure, the load detection is more easily affected by a change of a resistance at another crossing point in a case where a total number of measurement points becomes larger. In this case, an S/N ratio (signal noise ratio) becomes smaller, a measurement accuracy is deteriorated, and a peripheral circuit becomes more complicated in a case where a high-speed measurement is necessary and may be used in a robot.
JP2009-31045A discloses that a horizontal transistor including an organic semiconductor film made of an organic semiconductor material is applied to a load sensor, so as to solve the above matters. The horizontal transistor using the organic semiconductor film executes the load detection based on a change of a current Ids flowing between a drain electrode and a source electrode which is generated due to a change of a thickness of a gate oxidation film generated according to a load applied to the horizontal transistor. Since a current flows through the horizontal transistor until a gate is turned on, in the load sensor using the horizontal transistor, the horizontal transistor is not affected by other horizontal transistors adjacent to the horizontal transistor, the S/N ratio CaO be properly increased, and the measurement accuracy can be improved.
The current Ids is obtained by an expression based on a mobility μ, a gate capacity C, a gate width W, a gate length L, a gate voltage Vg, and a threshold voltage Vth. According to the above expression, when a thickness of the gate oxidation film is changed, parameters including the mobility μ, the gate capacity C, and the gate length L are affected. Therefore, the current Ids flowing between the drain electrode and the source electrode is changed in a non-linear relationship.
  Ids  =                    W        ×        μ        ×        C                    2        ×        L              ⁢                  (                  Vg          -          Vth                )            2      
In this case, to apply the horizontal transistor to the load sensor, a reference load is applied to the horizontal transistor, and the current Ids is measured and is established to a map and a table. That is, the current Ids of when a load is applied to the horizontal transistor is measured, a stored load in the table corresponding to the current Ids is loaded to measure the load. As the above description, when the current Ids is changed in a non-linear relationship to the load, it is necessary to establish a table by measuring the reference load, and a manufacturing of a product becomes complicated.